Excimer lasers have a capability to generate laser-radiation having a fundamental wavelength in the ultraviolet (UV) region of the electromagnetic spectrum. Operation of excimer lasers is based on an optical transition between different electronically excited states of noble-gas molecules. Such molecules exist only in an electronically excited state and accordingly are generally referred to as “excimers”. The noble-gas halide molecules are created in the excited state by a short and powerful electrical excitation (gas discharge) of between about 1 nanosecond (ns) and 1000 ns duration. Relaxation of the excited molecules to the ground state results in the emission of high-intensity UV light in a laser-resonator, which delivers the UV light as laser-radiation.
An excimer laser uses as an active medium (lasing gas) a noble gas, typically argon (Ar), krypton (Kr), or xenon (Xe), and a halogen-donating gas, typically hydrogen chloride (HCl) or fluorine (F2). These excimer forming gases are low concentration additives in a neutral buffer gas, typically neon (Ne) or helium (He). The buffer gas must be at a relatively high pressure, for example between about 2 atmospheres pressure (Bar) and about 10 Bar, in order to provide a desired impedance matching of the electrical gas discharge and adequate laser efficiency. The specific noble-gas and halogen-donating gas determine the wavelength of the laser-radiation.
The most powerful state-of-the-art industrial excimer lasers are based on xenon chloride (XeCl) molecules and generate laser-radiation having a wavelength of 308 nanometers (nm). Such excimer lasers can provide 308 nm laser-radiation pulses having a pulse-energy of about 1000 millijoules (mJ) at a pulse-repetition frequency (PRF) of about 600 Hertz (Hz). Such a laser is able to operate continuously over 100 million pulses while maintaining very high stability of pulse-energy and temporal and spatial optical parameters.
In laser-processes requiring greater pulse energy, the output of two or more such lasers can be combined by suitable beam-mixing optics and appropriate synchronization of pulse delivery. Such beam-mixing and synchronization are described in U.S. Pat. No. 7,408,714 and U.S. Pat. No. 8,238,400, respectively, each thereof assigned to the assignee of the present invention, and the complete disclosure of each of which is hereby incorporated herein by reference.
The duration of continuous operation of the laser is limited by degradation of the lasing gas and of some components of the laser, especially windows in a chamber containing the lasing gas and excitation (discharge) electrodes. Performance degradation can eventually develop due to chemical and electrical erosion of the discharge electrodes and other surfaces in the highly-reactive halogen-containing atmosphere in the chamber. Such erosion leads to contamination of the lasing gas and the laser chamber windows. As a result, interruptions of the laser operation are periodically needed to exchange the lasing gas and to service the laser windows.
The lasing-gas mixture typically used in such a laser consists primarily of Ne as the buffer gas at elevated pressure (for example about 6 Bar), with small additions of Xe (about 1%) and HCl (about 0.1%). Composition of the lasing-gas mixture is optimized in a way that allows an acceptable compromise between laser-efficiency and the other laser parameters.
Small additions of hydrogen, for example between about 10 and about 2000 parts-per-million (ppm), are commonly used to help stabilize excimer laser performance. Such small additions can significantly extend the lifetime of the lasing gas. However, an increase of the hydrogen concentration above an optimal concentration leads to a degradation of the laser performance and reduction of the laser output energy. A compromise between the lasing-gas lifetime and the output pulse-energy and stability determines the optimal concentration of hydrogen. This can depend on desired output parameters and other features of a particular laser.
One unfortunate characteristic of laser-radiation pulses delivered by an excimer laser is that each pulse is characterized by a first portion having a certain amplitude followed by a second portion having about half the amplitude of the first portion with a minimum amplitude between the first and second portions of the pulse. This is illustrated in FIG. 1, which is a graph schematically illustrating pulse amplitude as a function of time for a 308 nm XeCl excimer laser having a lasing-gas mixture composition as discussed above. It can be seen that the first portion of the pulse (portion A) has an amplitude more than twice that of the second portion of the pulse (portion B).
This characteristic, while relatively benign for certain laser-processing operations, such a cutting or ablation of materials, can be problematical for processes that depend more critically on the temporal characteristics of pulse energy delivery. One particular such process is excimer-laser recrystallization of silicon, which is a process used extensively in the manufacture of flat panels for large-screen displays. In this process, there is a particular energy density-per-pulse, generally referred to as the optimum energy density (OED) that produces, from an amorphous silicon layer, a poly-crystalline layer having a relatively uniform grain structure and having a minimum of defects that could adversely affect production yields of usable flat panels.
Problems of the above discussed two-portion pulse delivery have been substantially minimized by relatively recent developments in-situ, i.e., on a production line, monitoring of the recrystallizing process which can be arranged to adjust pulse-energy at least manually, responsive to optical characterization of flat panels being recrystallized. Such a monitoring process and characterization of the silicon recrystallization itself are described in detail in U.S. Pat. No. 9,335,276, and U.S Pub. No. 2013/0341310, both assigned to the assignee of the present invention, and with the complete disclosures therein are hereby incorporated herein by reference. It is believed, however, that improvements in the sensitivity and effectiveness of these processes may be possible if a means could be found to reduce the difference between the above-described first and second portions of excimer laser pulses.